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The melting of glaciers around the world is one of the hardest to ignore impacts of climate change (unless you don’t believe your eyes). While worries about rising sea levels are focused on the massive ice sheets of Greenland and Antarctica, the loss of small mountain glaciers comes with its own consequences. A pair of studies published this week highlight two such impacts—one extremely common, one extremely unusual.

Peak runoff

The first study, by Matthias Huss and Regine Hock, looks at the effect shrinking glaciers have on local water supplies. Glaciers help sustain rivers downslope through the drier months by providing a constant stream of meltwater, like a frozen water tower that collects in the winter and rations it out over the summer.

For a while, a shrinking glacier will contribute even more meltwater runoff to the river, but there comes a point when a smaller glacier can't keep up. Once it produces less meltwater, it’s downhill to “peak runoff” from there. This process has already been observed at a number of glaciers, but a global picture had not yet been painted.

So Huss and Hock turned to a model of mountain glaciers around the world. The model simulated each glacier from 1980 to 2100, using observed weather conditions for the past and a number of climate models for the future.

In the lowest greenhouse gas emissions scenario for the future (where warming stops about 1°C past present temperatures), the world’s mountain glaciers lose just over 40 percent of their mass by 2100. In the highest emissions scenario, nearly three-quarters of the mountains’ ice is lost. Larger glaciers hold out longer, but declining meltwater is inevitable. Counterintuitively, stronger global warming actually delays the peak by getting more meltwater from the ice as it's vanishing.

The really shocking number, however, is the number that have already passed “peak runoff” before the present day. That was true for 45 percent of the glacier-fed watersheds in the model. For these rivers, the summer glacier-water contribution is already decreasing.

In the middle emissions scenario, over 90 percent of glaciers cross that point by the end of the century. For half of the watersheds, the reduced meltwater means a decrease of summer streamflow under five percent, but a third of them lose more than 10 percent. There’s also a shift in timing. Generally, the supply of early-summer meltwater increases as warmer temperatures get glaciers melting sooner than they used to. The big decline comes later in the summer, which is often the time of year that reliable streamflow is needed the most.

So for areas counting on friendly neighborhood glaciers to provide water, change is coming—if it hasn’t arrived already.

A deadly alpine slip and slide

Another study—this one led by Andreas Kääb of the University of Oslo—looked at the avalanche-like collapse of two neighboring Tibetan glaciers in 2016. This event was remarkable, but we can’t say it’s unprecedented. In 2002, a glacier in the mountains that separate Russia from Georgia and Azerbaijan similarly collapsed, killing 120 people.

Here, the lower portion of a glacier in the Aru Mountains essentially broke off and slid six kilometers downslope, killing nine herders along with their livestock. When the event was examined, researchers noticed that the glacier one valley over was creased with large crevasses similar to those seen in earlier satellite images of its collapsed neighbor. As a result, a warning was issued that the second glacier could collapse. And collapse it did, just two months after the first. Fortunately, no one was hurt this time. But were the two events related? And why did these glaciers let go of nearly half their ice, anyway?

To answer these questions, the researchers launched a multi-pronged analysis. They worked with satellite images and measurements of the glaciers along with local weather data to understand what happened over the last few decades. They visited the glaciers to get up an up-close look at the aftermath. And they modeled them to work out how the likely mechanisms behind the collapses.

The team found that, like many glaciers in this region, these two experienced a combination of warming temperatures and increased precipitation in recent years. Warmer temperatures mean more melting at the downslope end of the glacier, but more snowfall means a build-up of mass upslope. If you deflate the lower half of the glacier and inflate the upper half, the surface of the glacier steepens—increasing the gravitational force pushing ice downhill.

The other half of the story is at the base of these glaciers. Some glaciers are frozen to the ground beneath them, while glaciers on unfrozen ground can slide along. The Aru glaciers probably straddle both categories, with the lower-most edges of the glaciers frozen but the central portion of its base thawed. Since glacial ice deforms like very slow putty, the steepening of the glacier would have caused the more mobile, middle section to flow pick up speed. But with the downslope end of the glacier frozen to the ground, that flow was held back like a dammed river.

As crevasses opened up in the ice behind the “dam," meltwater could trickle down to the base and accumulate just upslope of the frozen ground. Eventually, the building strain and seeping water caused the “dam” to fail. Between the upslope meltwater and the friction caused by the now-sliding lower section of the glacier, the previously frozen sediment beneath the glacier quickly turned in a slip-and-slide. And that’s how nearly half a glacier can be sent careening downhill like a runaway truck.

As for the link between the two collapses, there doesn’t seem to be anything except that they both experienced the same temperature and precipitation patterns. It’s very unlikely that the first collapse somehow triggered the second.

The researchers write that while these events are unusual, the conditions are not unique. By studying this collapse carefully, we should be able to identify other glaciers at risk—and hopefully provide life-saving warnings.